More than 1 million people were today ordered to evacuate Texas and Louisiana as Hurricane Rita - now downgraded to a Category 4 storm - bears down on the US Gulf coast.

Forecasters expect the storm, still blowing winds of 150mph, to make landfall on the central Texas coast early on Saturday morning. It will be one of the most intense hurricanes ever to hit the state. At 6pm BST today, it was just 435 miles south east of the Texan city of Galveston.

The storm's predicted course means it should miss New Orleans, but even a slight turn to the right would make it possible that it could breach the already damaged system of levees protecting the city.

The Army Corps of Engineers said the New Orleans levees would only be able to handle up to 6in (15cm) of rain and a storm surge of up to 12ft (3.6 metres).

Officials in Texas and Louisiana ordered the evacuations amid fears the devastation could be on a scale similar to that caused by Hurricane Katrina.

Still reeling from criticism that his administration had been too slow to respond to Katrina, the US president, George Bush, has declared a state of emergency in both states.

"Federal, state and local governments are coordinating their efforts to get ready," Mr Bush said. "We hope and pray that Hurricane Rita will not be a devastating storm, but we've got to be ready for the worst."

For a number of hours today, Rita was a Category 5 hurricane, the highest rating possible. Only three Category 5 hurricanes are known to have hit the US mainland, the most recent being Hurricane Andrew, which devastated south Florida in 1992.

"It's scary. It's really scary," Galveston resident Shalonda Dunn said as she and her young daughters waited to board a bus laid on by emergency authorities. "I'm glad we've got the opportunity to leave ... you never know what can happen."

In the Galveston-Houston-Corpus Christi area, around 1.3 million people were under orders to leave, along with a further 20,000 on the Louisiana coast.

Special attention has been given to hospitals and nursing homes, three weeks after dozens of sick and elderly patients in the New Orleans area drowned in Katrina's floodwaters or died in stifling heat as they waited to be rescued.

Rick Perry, the governor of Texas, said Galveston authorities were using computer projections to work out which areas of the city could be left submerged by flooding after being hit by the storm.

"Between Katrina and our preparations for this, people understand this isn't something you're going to play around with," Mr Perry told CNN.

Galveston was hit by one of the deadliest natural disasters in US history when an unnamed hurricane killed between 6,000 and 12,000 people and almost wiped the city off the map in 1900.

Around 5,000 National Guard troops are on standby in Texas, and 1,000 public safety officers have been posted along evacuation routes. Shelters for more than 250,000 refugees have been prepared in Huntsville, College Station, San Antonio and Dallas.

Harold Hurtt, the chief of police in Houston, warned anyone staying behind that looting would not be tolerated and anyone caught stealing after the storm would be prosecuted.

The death toll from Hurricane Katrina now stands at 1,036 with 799 confirmed dead in Louisiana alone.

Rita is the 17th named storm of the Atlantic hurricane season, making this the fourth-busiest season since records began in 1851. The record is 21 tropical storms in 1933, and the hurricane season finishes on November 30.